EP0390775A1

EP0390775A1 - A grindstone

Info

Publication number: EP0390775A1
Application number: EP88900370A
Authority: EP
Inventors: Fred Sunden; Carl-Olof Palm; Toivo Nurmi; Martti Keppo
Original assignee: Partek Oy AB
Current assignee: Partek Oy AB
Priority date: 1987-12-15
Filing date: 1987-12-15
Publication date: 1990-10-10
Also published as: WO1989005712A1; JPH03502554A; NO902651L; NO902651D0

Abstract

La présente invention se rapporte à une meule servant à produire de la pâte de bois. La surface de meulage est formée par des grains de meulage et par une matrice de ciment destinée à former l'agglomération des grains. La matrice comprend un mélange à base de ciment et la dureté du mélange à base de ciment par rapport aux grains est commandée de sorte que la surface de meulage s'aiguise automatiquement pendant l'opération de meulage. La présente invention se rapporte également à un procédé servant à fabriquer un mélange de coulée pour une meule. Ledit procédé consiste à mélanger de l'eau et un mélange à base de ciment Portland à faible dégagement de chaleur, résistant aux sulfates et à durcissement lent, le rapport eau/ciment étant compris entre environ 0,30 et 0,35; à effectuer la rhéologie du mélange de coulée au moyen d'un agent plastifiant à la place de l'eau excédentaire; et à effectuer le compactage du mélange par brassage, par vibrations, par compression ou par quelque autre moyen.The present invention relates to a grinding wheel for producing wood pulp. The grinding surface is formed by grinding grains and by a cement matrix intended to form the agglomeration of the grains. The matrix comprises a cement-based mixture and the hardness of the cement-based mixture relative to the grains is controlled so that the grinding surface is automatically sharpened during the grinding operation. The present invention also relates to a method for manufacturing a casting mixture for a grinding wheel. Said method comprises mixing water and a mixture of Portland cement with low heat generation, sulphate resistant and slow curing, the water / cement ratio being between approximately 0.30 and 0.35; performing the rheology of the casting mixture by means of a plasticizing agent in place of the excess water; and compacting the mixture by stirring, vibration, compression or some other means.

Description

A grindstone

The present invention relates to a grindstone intended for the production of groundwood pulp, the grinding surface of said grindstone being substantially formed by grinding grits and a concrete matrix binding the grits together.

In the manufacture of paper pulp it has long been known to use a rotating grindstone cast of concrete for mechanically separating fibres from each other. In principle, such a grindstone is formed as a body of reinforced concrete and hard grinding grits embedded evenly therein. Conventional Portland cement has been used as a binder and arenaceous quartz as grinding grits. Normally, a concrete cylinder is attached to a rotating shaft by clamping it at the end faces thereof between two flanges. A grindstone structure of this type has been used throughout the 20th century and it is still partly in use. Pulpwood logs are introduced through a verrical pit against the shell of the grindstone so that they make contact with the shell over the whole length thereof due to the weight of the wood contained in the pit. This process, still in use, is known as a GW process.

With increasing production requirements, a grindstone structure of the above type has no longer proved advantageous. For obtaining the necessary strength, the manufacture of the grindstone has to be carried out very carefully; for instance, it is necessary to store a grindstone for two to three years before it is taken into use in order that strains caused by the shrinking of concrete would die out. For achieving a sufficient strength, it is also necessary to use relatively great amounts of cement, which, in turns, requires greater amounts of water than usual for achieving a good castability. The excess water deteriorates the strength of the structure and increases the shrinking and the formation of strains. The concrete surface of the grindstone wears out relatively rapidly in the grinding process because it has not been possible to obtain a cement composition strong enough to hold the protecting grinding grits. As a result of this the grinding grits come off prematurely, i.e. when still capable of grinding, leaving the softer concrete surface free for wear. Consequently, the sharp local edges formed on the grinding surface grind the logs rather into "sawdust" than into desired fibres. In addition, a conventional concrete grinding shell is not particularly resistant to the effect of the water used in the process. Especially sulphate ions contained in the water affect adversely the concrete. For these reasons, it is necessary to sharpen the grindstone, i.e. provide it with grooves of a determined shape, even three to five times in a day.

The above drawbacks have become even more critical in new methods increasing the production rate. In these methods, the pulpwood logs are introduced and pressed against the shell of the grindstone over the whole length thereof in closed chambers by means of hydraulically operated pistons. Fibres so obtained are sprayed off with process water directed to the surface of the grindstone and having a temperature of up to 140°C on account of the increased pressure. The high temperature of the water contributes to the formation of fibres. A grinding process of this type is known as a PGW process, in which the grindstone is subjected to strains considerably greater than in the earlier GW process. The concrete stone described above is practically unusable in a PGW process.

For the elimination of these drawbacks, the composition and structure of the grindstone have been under study. As to the first-mentioned, the properties of the material binding together the grinding grits is of crucial importance. From now on, this structural part of the grindstone will be called a matrix. More specifically, a matrix is a physico-chemical structural part of a grindstone that binds together the grinding grits and the possible reinforcements into a strong grindstone functioning as an integral body. In the grindstone described above, the matrix is formed by concrete based on Portland cement. In other known grinding methods, the matrix is formed by ceramic materials. The shell of a concrete grinding drum is thereby covered with grindstone segments about 200 x 200 x 300 mm in size. These consist of a matrix bound by sintering, which matrix, in turn, binds together the grinding grits. Since the wearing strength of a ceramic material is considerably greater than that of ordinary concrete, it is economical that the grinding grits embedded in a ceramic grindstone are considerably harder than quartz, such as the known Al₂O₃, SiC, etc. Ceramic segments are usually fastened to the drum by means of a hook or loop provided therein by embedding the hook or loop in the load-bearing mid portion of the concrete drum. Each segment should be able to receive forces exerted thereon in the peripheral direction, wherefore interspaces between the segments are filled with a flexible and force absorbing material.

An advantage of a ceramic grindstone is the high wear resistance thereof. Instead of coming off easily and prematurely, the grinding grits in general stay in place until they wear out. After the grinding grits have worn to the level of the matrix, the grindstone is no longer effective, and in order to restore the formation of fibres the grindstone has to be sharpened, i.e. provided with grooves having a determined shape, similarly as in the case of a concrete grindstone. The sharpening has to be repeated about every other week which is a major improvement as compared with a concrete stone. Moreover, the ceramic shell is not liable to the disad vantageous effects of the process water, but withstands well the corrosive effect of the concrets corroding substances contained therein. It has also been found out that a ceramic grinding shell retains its circular cross-section better than a concrete grindstone. Unnecessary much material has to be removed from a concrete grindstone for retaining the circular shape. For this reason, among others, a concrete grindstone may wear out in six months, whereas a ceramic shell may endure for about two to four years.

Despite the high wearing strength of a ceramic grindstone, it does not fulfil all the requirements of the present-day large-scale production. Being of a ceramic material, it is fragile. It cracks easily, and since it is very difficult to replace individual segments, the entire drum has to be replaced after the cracking of a determined number of the segments, For a major part, the grinding is carried out by means of inferior grindstones decreasing the production rate. For the missing toughness, a ceramic grindstone does not withstand any greater concentrated or linear loads, such as loads transmitted through the fastening means of the shell during the deformation of the shell. No unnecessary strains are allowed, wherefore the temperature of the grindstone, for instance, has to be elevated to the operating temperature very cautiously and kept at this temperature rather accurately. Due to the great mass thereof, a grindstone is liable to temperature variations, and the unequal coefficients of heat expansion of the concrete body, the ceramic segments and the fastening irons thereof cause disadvantageous relative movements and strains to occur between the different parts. At worst, the ceramic shell may be wholly stripped off in case of instantaneous excessive load.

In spite of these drawbacks, ceramic grindstones are today used almost without exception in the grinding of wood.

The object of this invention is to provide a new and improved grindstone, which combines the advantages of the concrete and ceramic grinding elements without taking over the disadvantages. The grindstone according to the invention is mainly characterized in that the matrix is formed by a cement mixture containing slow-hardening, sulphate-resisting, low-heat Portland cement, slag cement or the like, strength-increasing silica additive and plasticizing agent, and that the grinding grits are of a material having a hardness ≥ 2,000 Vickers, whereby the hardness of the cement matrix in relation to the hardness of the grinding grits is so controlled that the grinding surface is at least substantially self-sharpening during the grinding process.

The grinding grits preferably consist of sole aluminium oxide or silicon carbide grits or conglomerates thereof.

Contrary to ceramic grindstones, the matrix of the grindstone according to the invention may further contain e.g. metallic reinforcing fibres, which is a very important advantage, giving the matrix an excellent toughness compared to the brittle constitution of ceramic materials. That portion of the grindstone which forms the grinding surface preferably contains slow-hardening, sulphate-resisting low-heat Portland cement, slag cement or the like in an amount of about 25 to 35% by weight, silica fume or the like about 5 to 8% by weight, a plasticizing agent about 1 to 2% by weight, grinding grits about 50 to 65% by weight and reinforcing fibres about 0 to 10% by weight.

In the grindstone according to the invention, the grinding surface is preferably formed as segments, similarly as in ceramic grindstones. Distinct from a ceramic grindstone having a concrete body, the grindstone segments according to the invention can be secured to the body portion, e.g. a hollow cylinder of steel, from the outside by means of threaded bars or the like inserted through the segments and embedded therein and pinching nuts attached thereto, whereby it is not necessary to fill the interspaces between the segments.

The new type of grindstone concrete matrix according to the invention combines the advantages of prior concrete and ceramic matrices without the drawbacks of prior matrices. The advantages of the matrix are specified as follows:

- an extremely dense structure, i.e. little if any strength-reducing pores;

- a great strength as well as a tough structure; - in the form of a segment, high resistance to concentrated and linear loads caused by the fastening means thereof;

- optimized wear resistance with respect to the properties of the used grinding grits so that the matrix and the grinding grits wear out in an advantageous proportional manner with respect to each other for the formation of a fibre as good as possible without any sharpening and blunting routines;

- resistant to the corrosive effect of the process water;

- separately replaceable segments and the interspaces at the joints of the shell formed by the segments need not necessarily be filled with a material balancing the forces. These advantages have been accurately proved in tests carried out. In practice, a property of great importance to the production is the self-sharpening property of the grinding surface which enables the grinding process to be carried out substantially continuously until the grinding segments are thoroughly worn out. There are some highly surprising results probably also to be attributed to the self-sharpening property, i.e. that the specific consumption of energy as compared with a ceramic grindstone was reduced as much as 20 to 30%, and, in increase of 25 to 40% was obtained in the production; and the so called chest shive content, i.e. the amount of groundwood containing inferior fibres, was reduced about 40%. The self-sharpening property can be explicitly explained by the fact that when the initially sharp grinding grits wear out and get round, the matrix simultaneously wears in an advantageous and structurally controlled manner so that it releases continuously new grinding grits. In this way the sharp grinding surface advantageous for the fibre formation remains unchanged. This is not possible with a ceramic matrix, because it is throughout too hard. The increase in the production rate, i.e. the increase in the amount of acceptable fibre per time unit, is probably to be explained on the same grounds.

The solution according to the invention is based on the main points of cement chemistry well-known per se. When conventional ordinary cement reacts with water, plenty of calcium hydroxide (Ca(OH)₂) is formed at the initial stage. This is a reaction product of tricalsium silicate (C₃S) and dicalcium silicate (C₂S) contained in cement. The content of the first-mentioned in ordinary cement is about 60% by weight and that of the last-mentioned about 20% by weight. Another reaction product obtained in these processes is hydrated calcium silicate (3CaO·2SiO₂·3H₂O), also known by the name Tobermorit. This is the most important final product of the hardening process, and it determines the properties of the finished concrete. Calcium hydroxide, in turn, has a low strength due to its crystal structure (cleavage planes), and is soluble and very reactive in acidic and sulphatic conditions. The pH of the spray water used in the grinding process is about 4.5 to 5; a relatively high temperature, about 100°C, intensifies the reaction. Consequently, water dissolves calcium hydroxide from the concrete, i.e. pores are formed. Sulphates contained in the spray water are also liable to react with Ca(OH)₂ so that an expansive component called ettringite is formed. Other components of cement also participate in the hardening process, but these are not of primary importance in view of the present invention. In the manufacture of ordinary concrete, the amount of water in relation to the amount of cement is tried to be kept as small as possible for the achievement of good strength properties, whereby the rheology of the casting mixture is bad. The amount of water has to be increased for improving the rheology, which is necessary e.g. in the manufacture of concrete grindstones. However, excess water that is not needed in the hardening process is formed and gradually evaporated after the hardening, which leads to the formation of strength-reducing pores.

In the solution according to the invention the object has been to affect all parametres determining the properties of the matrix. The selected hydraulic binder is inexpensive and advantageous in strength. A well- known binder called LHSR cement with slow strength development and low evolution of heat is preferably used as a binder. These properties ensure that the state of ini tial strain formed in connection with the casting nearly nonexistent. The percentages by weight of C₃S and C₂S contained therein are about 20% and 60%, respectively, i.e. the reverse as compared with ordinary cement. For this reason, disadvantageous calcium hydroxide is formed considerably less than in the case of ordinary cement: C₂S produces this disadvantageous substance in an amount only half of the amount produced by C₃S. Due to the low tricalcium aluminate 3CaO₂·Al²O₃ content thereof (<2%), this cement also gives the concrete an excellent sulphate resistance.

According to the invention, disadvantageous Ca(OH)₂ is removed in such a way that it is replaced with components reinforcing the matrix, i.e. strong silicate bridges combining the particles of the matrix. Calcium hydroxide does not only form a cover around the cement particles but obviously also around the grinding grits. The cavities formed therebetween are also partly filled with calcium hydroxide and partly with the excess water which is gradually evaporated. According to the invention the calcium hydroxide is replaced and the water amount used is considerably reduced and replaced volumetrically with silica fume. Trades names include MICROPOZ" with the composition Si0₂_86 to 96%; C:≤2.0%; Fe₂O₃: ≤ 2.2%; Na₂O: ≤ 1.8%; Al₂O₃: ≤ 2.2%; K₂O: ≤ 2.5%; MgO: ≤ 2.O%; and SO₃: ≤ 1.5% (annealing loss ≤ 3.0%). Silica fume is known per se as a concrete additive, and its grain size is smaller than 0.5 μm and, accordingly, it has a very large specific area. By virtue thereof, it has a high water binding ability. When metered suitably in relation to the amount of water, it thus binds and removes the water which stoichiometrically is not needed in the hardening process. It reacts with water ana calcium hydroxide, thus forming strong Tobermorit bridges between the grinding grits. As to the grinding surfaces. similar to crystal surfaces, this mechanism has not yet been proved; on the other hand, it is known that this kind of binding takes place e.g. on the crystal surfaces of metals, so there is reason to assume that the binding mechanism applies to grinding grits as well. In any case it has been found, that by proportioning the amounts of silica fume and water according to the invention to the desired properties of the matrix in question, the weak calcium hydroxide crystals in the matrix so obtained have been converted into Tobermorit bridges. No pores formed by water occur in the matrix; instead, these are replaced with a component which considerably improves the strength properties. The matrix is very compact.

Reduction in the amount of water normally deteriorates the rheology and castability of the mixture to be cast. This is a drawback in that the mixture does thereby not spread efficiently around irregular grinding grains or into the cavities therebetween. Voids filled with water or air may be formed. To avoid this, the consistence of the casting mixture is improved by adding thereto a plasticizing agent known per se, sold e.g. under the names PARMIX, MIGHTY, MELMENT, etc. If required, it is also possible to carry out the casting of the grindstone in vacuum by stirring, compressing or vibrating the casting mixture so that air is removed and the mixture is compacted. If needed, it is also possible to add to the mixture suitable, e.g. metallic reinforcing fibres in order to improve the toughness of the grindstone. This is not possible with ceramic grindstones, because the metallic fibres would melt.

Another improvement according to the invention relates to the structure of the grinding grits. Naturally, a small grit does not stick to the matrix as well as a big one. Grinding grits of utmost size may, on the other hand, be teared off premataurely, i . e . before they have lost their grinding ability.

According to a preferred embodiment of the present invention, this drawback is overcome by using alternative conglomerates of grinding grits. As said before, ceramics have excellent wear resistance and binding features. By sintering together grinding grits with ceramics, the weak binding due to small contact surfaces is compensated by an excellent binder and the weak binding to the concrete matrix is compensated by a larger contact surface. The conglomerates are optimally distributed in the matrix to obtain natural grooves, for selfsharpening and for receiving the fibres ground off, as explained before. The self-sharpening of the grindstone is obtained by controlling the proportional wear of the matrix and the conglomerates. Practically, this is achieved by optimally distributing individual grinding grits between the conglomerates to form the desired groove paths.

According to a preferred embodiment of the invention the conglomerates are given a rodlike shape. Each rod may then extend through the whole radial depth of the grinding segment or at least along a part of it.

In the following the invention will be described with reference to the attached drawing. Figure 1 is a partial view of one embodiment of the invention in a cross-section.

Figure 2 is a top view of a part of a grinding surface formed by grinding segments.

Figures 3 and 4 illustrate differences between a grindstone according to the invention and a ceramic grindstone in view of the wear of the grinding surface and the energy consumption, respectively .

Figures 5 and 6 are strongly enlarged views of the surface of a ceramic grindstone and the grindstone according to the invention, respectively. Figure 7 illustrates a conglomerate embodiment, including the rodlike alternative, and small grits, distributed between these.

In principle, the grindstone according to the invention could be entirely formed by a one-piece matrix and grinding grits, but this is uneconomical in most cases. Alternatively, the matrix and the grinding grits. thereof could form a hollow cylinder within which a body portion of ordinary concrete is arranged. It would, also be possible to attach grinding segments to such a body portion similarly as in prior ceramic grindstones.

Figures 1 and 2 show schematically an embodiment advantageous over these.

The body portion of the grindstone is formed as a hollow steel cylinder 1 which may be closed at the ends and attached to an operating shaft. The grinding surface consists of grinding segments 2 each one of which is fastened to the metal cylinder 1 e.g. by means of two bolts 3 to be inserted through the segment 2 from the outside. The bolts 3 are anchored in the cylinder 1 and the segments 2 are tightened in place by means of nuts 4. The nuts 4 are sunk in recesses 5 formed in the surface of the segments 2, so that the nuts can be reached from the outside, whereby the segments are easily replaceable separately, if required. There is no need to position a flexible supporting agent or a medium in interspaces 6 between the segments 2 as in the case of ceramic grindstones, instead, these can be open. The interspaces 6, similarly as the recesses 5, are thereby able to receive fibres ground off so as to avoid any unnecessary grinding of the fibres on the grinding surface. This is a major advantage over grindstones provided with a concrete body and ceramic segments. When the grinding surface is provided with grooves, fibres ground off will get into these grooves so that they will not be exposed to constant breaking pressure between the grinding surface and the logs. The spray water flushes them out of the grooves. In this respect a ceramic grinding surface is relatively smooth, and it takes a lot of time and requires special equipment to make the grooves sufficiently deep in connection with the sharpening. The optimized matrix according to the invention enables the grinding surface and the grinding segments to be provided with advantageous recesses for this purpose as early as at the casting stage. The recesses of the fastening bolts, for instance, are advantageous in this respect. During the grinding new grooves are shaped due to the selfsharpening process.

Another major advantage of the invention over ceramic segments is the direct contact between the fastening bolts 3 and the spray water. By virtue of the greater thermal conductivity of the bolts, it is easier to remove any undesired heat from the grindstone structure than from a ceramic structure in which the material between the bolts and the spray water has a lower thermal conductivity. The grinding segments 2 can, naturally, be positioned arbitrarily with respect to each other. One example would be the embodiment similar to a masonry wall shown in Figure 2. In the example of Figure 2, the recesses 5 are connected to the interspaces 6 and to each other by means of grooves 7. Of course a lot of other configurations can be obtained.

Example

Square grinding segments 150 × 150 × 70 mm in size and having a matrix according to the invention were manufactured. Each segment was fastened to a cylindrical steel body by means of two bolts going through the segment in such a way that a cylindrical grinding shell having a thickness of 70 mm, and outer diameter of 980 mm and a length of 660 mm was formed around the drum. No kind of medium for balancing the peripheral forces was provided in the interspaces formed at the joints. This grindstone was rotated in a PGW grinding process at a peripheral speed of 35 m/s, while the temperature of the spray water was 105°C and the pressure exerted on the logs 250 kPa.

The composition of the stone was as follows: Cement 33% by weight

Grinding grits, Al₂O₃ 58% " Silica fume (MICROPOZ ) 8% "

Plasticizing agent 1% "

Figure 3 illustrates the self-sharpening property of the grindstone according to the invention as compared with a ceramic grindstone in a set of time-production rate coordinates. The unbroken curve 8 represents a stone according to the invention and the broken-line curve 9 a ceramic stone.

Initially, i.e. after the sharpening, a ceramic grindstone requires a "running-in" of a couple of days, during which time the production rate is low. After a fully efficient grinding for about two weeks, the groundwood begins to get excessively fine, on account of which the grindstone has to be sharpened again.

The initial time required for the running-in of the grindstone according to the invention is practically nonexistent, the production rate is clearly higher than that of a ceramic grindstone and tests carried out by now confirm that no resharpening is needed.

When considering the features of the object of invention it should also be kept in mind, that the steel/concrete combination exhibits a good interaction as to relative movements between the integral parts. It is well known from the structural engineering that the concrete and the steel reinforcement have almost the same coefficient of thermal expansivity. This means, that the grinding stone according to the invention is not sensitive to temperature shocks or temperature variations in general, as consisting of concrete and steel only. A ceramic stone, on the contrary, is hypersensitive to temperature variations and thermal stress, since it consists of materials not interacting in an advantageous way. This explains the careful treatment of ceramic grinding stones, mentioned on page 4.

Figure 4 illustrates, in more detail as compared with Figure 3, the obtained energy savings, i.e. the increase in the production rate in a set of coordinates representing the specific energy consumption and the degree of fineness. The unbroken curve 10 represents the new grindstone, the broken-line curve 11 a ceramic grindstone. The energy consumption per one ton of groundwood is about 20 to 25% smaller when using the grindstone according to the invention.

Figure 5 illustrates the surface of the ceramic grindstone and Figure 6 the surface of the grindstone according to the invention in a strongly enlarged view. The matrices are designated with the reference numerals 12 and 13, respectively; the grinding grits with the reference numerals 14 and 15; and the fibres with the reference numeral 16. In Figure 5, the dark areas designates with the reference numeral 17 represent empty space, which does not occur in Figure 6, illustrative of the invention. In Figure 7, a part of a curved segment surface, the conglomerates are marked with a reference numeral 18, the rodlike alternatives with 19 and the wear controlling individual grits with 20. 21 designates individual grains in the conglomerate and 22 the ceramic binder. 23 designate grooves.

Claims

, Claims

1. A grindstone intended for the production of groundwood pulp, the grinding surface of said grindstone being substantially formed by grinding grits and a concrete matrix binding together the grits, c h a r a c t e r i z e d in that the matrix is formed by a cement mixture containing slow-hardening, sulphate-resisting, low-heat Portland cement, slag cement or the like, strength-increasing silica additive and plasticizing agent, and that the grinding grits are of a material having a hardness ≤ 2,000 Vickers, whereby the hardness of the cement matrix in relation to the hardness of the grinding grits is so controllled that the grinding surface is at least substantially self-sharpening during the grinding process.

2. A grindstone according to claim 1, ch a r a c t e r i z e d in that the cement mixture is of a low-heat, sulphate-resisting kind, the tricalcium silicate content thereof being no more than about 30% by weight and the tricalcium aluminate content no more than about 2% by weight.

3. A grindstone according to claim 1, c h a r a c t e r i z e d in that the matrix further comprises reinforcing fibres, preferably metallic fibres.

4. A grindstone according to claim 1, c h a r a c t e r i z e d in that the grinding grits comprice grits of aluminium oxide or silicon carbide.

5. A grindstone according to claim 1, c h a r a c t e r i z e d in that the grinding grits comprice conglomerates of distinct grinding grits bond together by ceramics.

6. A grindstone according to claim 5, c h a r a c t e r i z e d in that the conglomerates are of a rodlike shape throughout the whole depth of the grinding segment, or at least a part thereof.

7. A grindstone according to any of the claims 1 to 6, c h a r a c t e r i z e d in that the portion thereof forming the grinding surface comprises slow-hardening, sulphate-resisting low-heat Portland cement, slag cement or the like about 25 to 35% by weight silica fume or the like about 5 to 8% " plasticizing agent about 1 to 2% " grinding grits about 50 to 65% " reinforcing fibres about 0 to 10% "

8. A grindstone according to the claims 5 to 7, c h a r a c t e r i z e d in that wear controlling grinding grits are distributed between the conglomerates.

9. A grindstone according to claim 1, c h a r a c t e r i z e d in that the grinding surface thereof comprices segments (2) fastened to a body portion (1) of the grindstone from the outside by means of bolts (3) or the like inserted through the segments (2) and sunk therein.

10. A grindstone according to claim 9, c h a r a c t e r i z e d in that the body portion of the grindstone is a hollow metal cylinder (1).

11. A grindstone according to claim 9, c h a r a c t e r i z e d in that there is no medium provided in interspaces (6) defined between the segments (2).

12. A grindstone according to claim 11, c h a r a c t e r i z e d in that grooves (7) are formed on the surface of the segments (2) for joining the sinking recesses (5) of the bolts (3) or the like and the interspaces (6) defined between the segments.

13. A grindstone according to claim 1, c h a r a c t e r i z e d in that the grinding surface thereof is integral.

14. A method for the manufacture of a casting mixture for a grindstone intended for the production of groundwood pulp, c h a r a c t e r i z e d by the steps of mixing together water and a slow-hardening, sulphate-resisting low-heat Portland cement mixture having a tricalcium silicate content of no more than about 30% by weight and a tricalcium aluminate content of no more than about 2% by weight so that the water/cement ratio is no more than about 0.30 to 0.35; effecting the known effect of excess water necessary for the consistence of the casting mixture by a plasticizing agent instead of said excess water; and compacting the mixture in connection with the casting by stiring, vibrating, compressing or in some other way.

15. A method according to claim 14, c h a r a c t e r i z e d in that the mixing and casting of the mixture is carried out in a vacuum.